The present invention relates to the magnetic resonance arts. It finds particular application in dual contrast fast spin echo imaging techniques and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find application in conjunction with other types of multiple echo imaging techniques, particularly techniques with extended echo trains following each excitation.
In magnetic resonance imaging, the echo time, i.e., the time between excitation and the center of the data generating echo, is determinative of significant characteristics of the resultant image. For most imaged tissues, relatively short echo times provide stronger contrast and provide images that are more heavily T1 weighted. As the echo time becomes longer, the resultant images become more heavily T2 weighted. Short echo times cause the image to emphasize tissue with relatively short relaxation times and to emphasize T1 magnetization relaxation; whereas, relatively long echo times emphasize tissue with longer relaxation times and T2 relaxation. Thus, different echo times give different contrasts between various types of tissue within the image. Often, the echo time is selected in order to maximize the contrast between the tissues of interest and to select a relative T1/T2 weighting.
In spin echo imaging, resonance is excited followed by application of a 180.degree. refocusing pulse to induce a first echo. In fast spin echo imaging, the echo is followed by another 180.degree. refocusing pulse to induce a second echo which is followed by another 180.degree. refocusing pulse to induce a third echo, etc. In dual contrast fast spin echoes, the data from a first half of the echo train are used to generate a first image and data from the second half of the echo train are used to generate a second image. In this manner, the effective echo time of the first image is much shorter than the echo time of the second image, hence the two images have different contrasts.
Dual contrast fast spin echoes provide meaningful results when the echo train is relatively short. However, as the echo train becomes longer, the minimum effective echo time of the second image cannot be any shorter than the echo time of the first echo of the second half of the echo train. For long echo trains, such as 64 echoes, 128 echoes, or the like, the effective echo time of the second image becomes so long that the second image is of limited or no diagnostic value. Note that when the effective echo time exceeds the T1 relaxation time and approaches the T2 relaxation time of tissue of interest, the tissue of interest becomes faint or even disappears from the resultant image.
The present invention provides a new and improved imaging technique and apparatus in which data from relatively long echo trains can be used to generate two (or more) images, both with relatively short effective echo times.